Publication 1756-UM050A-EN-P - December 2001
Before You Configure Your Module 3-5
Subnet Mask
Subnet addressing is an extension of the IP address scheme that
allows a site to use a single net ID for multiple physical networks.
Routing outside of the site continues by dividing the IP address into a
net ID and a host ID via the class. Inside a site, the subnet mask is
used to redivide the IP address into a custom net ID portion and host
ID portion.
Take Network 2 (a Class B network) in the previous example and add
another physical network. Selecting the following subnet mask would
add two additional net ID bits allowing for four physical networks:
11111111 11111111 11000000 00000000 = 255.255.192.0
Two bits of the Class B host ID have been used to extend the net ID.
Each unique combination of bits in the part of the host ID where
subnet mask bits are 1 specifies a different physical network.
The new configuration is:
A second network with Hosts D and E has been added. Gateway G2
connects Network 2.1 with Network 2.2. Hosts D and E will use
Gateway G2 to communicate with hosts not on Network 2.2. Hosts B
and C will use Gateway G to communicate with hosts not on Network
2.1. When B is communicating with D, G (the configured Gateway for
B) will route the data from B to D through G2.
128.1.0.1
128.2.64.1 128.2.64.2
128.2.64.3
128.1.0.2
A
B
C
G
Network 1
Network 2.1
128.2.128.1 128.2.128.2
128.2.128.3
D
E
G2
Network 2.2
AB Parts
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